Get better span name when handler is a proxy

[trask]

Copied from @laurit's #3798 (comment)

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Getting a span name from an object is also used in other instrumentations so it would be nice if someone figured out how to do this properly. The problem here is that handler could be anything. For example SpringWebFluxTestApplication when you add

  @Component("greetingHandlerFunction")
  @Scope(value = "prototype", proxyMode = ScopedProxyMode.TARGET_CLASS)
  static class GreetingHandlerFunction implements HandlerFunction<ServerResponse> {
    private final GreetingHandler greetingHandler

    GreetingHandlerFunction(GreetingHandler greetingHandler) {
      this.greetingHandler = greetingHandler
    }

    @Override
    Mono<ServerResponse> handle(ServerRequest request) {
      return greetingHandler.defaultGreet()
    }
  }

and change the start of greetRouterFunction to

  @Bean
  RouterFunction<ServerResponse> greetRouterFunction(GreetingHandler greetingHandler, HandlerFunction<ServerResponse> greetingHandlerFunction) {
    return route(GET("/greet"), greetingHandlerFunction

Then span name is
SpringWebFluxTestApplication$GreetingHandlerFunction$$EnhancerBySpringCGLIB$$d662c0c3.handle
When you change scope annotation to
@Scope(value = "prototype", proxyMode = ScopedProxyMode.INTERFACES)
then span name is
$Proxy119.handle
If we'd assume that handler is a spring bean we could try to unwrap it with something like spring-projects/spring-framework@efe3a35

[trask]

Closing, these spans are now opt-in under an experimental flag, can revisit later if anyone has renewed interest.